1. Field of the Invention
This invention relates in general to methods and devices for reducing interference with wireless communications, and in particular, to a method and apparatus for reducing intermodulation interference in mobile cellular telephones.
2. Description of Related Art
Currently, much of the infrastructure for cellular telephone systems in the United States comprises AMPS (advanced mobile phone system) or analog-type cellular telephone systems. Next-generation CDMA (code division multiple access) cellular telephone systems are now being installed, often in parallel with AMPS cellular telephone systems. These CDMA cellular telephone systems rely on low power broadband spread spectrum transmission techniques. In most circumstances, CDMA and AMPS cellular systems can coexist without significant interference. However, recent experience has shown that interference between CDMA and the AMPS systems is possible under some circumstances. For example, when a CDMA mobile unit approaches an AMPS-only site, the AMPS signal overloads the CDMA mobile cellphone receiver radio frequency (RF) front-end circuits due to the relative strength of the AMPS signal and the relative weakness of the spread-spectrum CDMA signal. Intermodulation (IM) components created by the AMPS signal after low noise amplification (LNA) also interfere with the weak CDMA signal.
Efforts to curb this problem have focused on modifying the CDMA mobile units by the conventional method of adding attenuation pads and changing the range of LNA to reduce IM problems. Unfortunately, these changes are costly and difficult to implement because they rely on design changes to the mobile cellphones, which are large in number. Further, even where this technique has been used, it has not substantially improved reception quality.
Thus, there is a need for an inexpensive and effective system and method for eliminating intermodulation interference in cellular telephone systems.